The intermediate stage evolves from a state of maximal energy density.
The values achieved depend on the collision system and energy as well as on the impact parameter.
In the most general case, the energy density is not constant across the interaction volume but will drop towards its periphery and will exhibit fluctuations.
However, there is good evidence that throughout the evolution during this stage the system is locally equilibrated.
The true nature of the “thermalization” process leading to this situation is still a matter of research.
In particular, at lower beam energies, rapidity distributions of protons suggest that the fireball is not fully equilibrated.
Yet, it is conceivable that different degrees of freedom, e.g. the baryon excitation spectrum versus the baryon momentum distribution, will have different equilibration times.
Fast thermalization in general can be achieved if three or many-body collisions are at work, or, arguing in the language of quantum mechanics, if a certain degree of entanglement is realized such that the combined wave function of a number of constituents probes the phase space according to the level density available (Fermi's golden rule).
At the high-energy frontier, recently a pre-equilibrium stage has been introduced, which connects the initial stage characterized by multi-gluon interactions, to a later stage, when the system can be safely treated as hydrodynamic evolution.
This stage describes the evolution of the system to equilibration using kinetic theory for gluons [1]}.
